1. Field of the Invention
The present invention relates to an exhaust gas purification system of an internal combustion engine. Specifically, the present invention relates to a technology for regenerating a particulate filter.
2. Description of Related Art
In recent years, reduction of exhaust emission of internal combustion engines installed in automobiles and the like has been required. Specifically, in a compression ignition diesel engine using light oil as fuel, it is required to eliminate not only gases such as carbon monoxide, hydrocarbon and nitrogen oxides, but also exhaust particulate matters such as soot and soluble organic fractions contained in exhaust gas. Therefore, a particulate filter is disposed in an exhaust passage for collecting the exhaust particulate matters in the exhaust gas.
If the exhaust gas flows into the particulate filter, the exhaust gas passes through porous partition walls of the particulate filter. At that time, the exhaust particulate matters contained in the exhaust gas are collected at surfaces and pores of the partition walls. If a quantity of the collected and deposited particulate matters increases excessively, a flow resistance in the particulate filter will increase and a back pressure of the engine will increase. As a result, an output of the engine will be deteriorated. Therefore, the exhaust particulate matters deposited on the particulate filter should be occasionally eliminated from the particulate filter to regenerate the particulate filter.
A certain system capable of performing the regeneration of the particulate filter during operation of the engine utilizes an oxidizing property of an oxidation catalyst such as platinum disposed on the particulate filter. The system performs post-injection for supplying the fuel to the particulate filter in an exhaustion stroke, for instance. Thus, the system oxidizes and eliminates the deposited exhaust particulate matters, which are more difficult to oxidize than the injected fuel, by utilizing combustion heat of the fuel.
If the regeneration of the particulate filter is performed frequently, a fuel cost will increase. If a time interval to the next regeneration is too long, the quantity of the deposited exhaust particulate matters will increase excessively, and the deposited exhaust particulate matters will be suddenly burned in the regeneration process. In such a case, there is a possibility that the particulate filter is heated to abnormally high temperature and is damaged. Therefore, regeneration timing of the particulate filter should be preferably decided by determining a deposition state (a deposited quantity) of the exhaust particulate matters. Flow resistance in the particulate filter increases as the quantity of the exhaust particulate matters deposited in the particulate filter increases. A differential pressure between an inlet and an outlet of the particulate filter increases if the flow resistance increases. Therefore, a system disclosed in Unexamined Japanese Patent Application Publication No. H07-332065 (Patent Document 1) measures the differential pressure across the particulate filter, and determines that the timing for performing the regeneration is reached if the differential pressure exceeds a predetermined value. The differential pressure of the particulate filter changes in accordance with a flow rate of the exhaust gas flowing through the particulate filter. Therefore, a system disclosed in Unexamined Japanese Patent Application Publication No. H07-317529 (Patent Document 2) converts the measured differential pressure into a value corresponding to a predetermined flow rate of the exhaust gas. The technology of Patent Document 2 calculates the present flow rate of the exhaust gas, which is necessary in the conversion, based on a cylinder volume, engine rotation speed and exhaust gas temperature of the engine.
However, a flowing state of the exhaust gas in the exhaust passage, through which the exhaust gas flows, is complicated. Therefore, there is a possibility that the technology of Patent Document 2 cannot properly determine the regeneration timing.